With popularization of information terminals, there has been a growing demand for flat panel displays as computer displays. Moreover, with progress in information technology, information conventionally provided on a paper medium has been increasingly provided as electronic data. Electronic paper or digital paper has therefore been increasingly demanded as a thin, lightweight, easily portable mobile display medium (for example, PATENT DOCUMENT 1).
Generally, in flat panel displays, display media are formed by using elements utilizing liquid crystal, organic EL (organic electroluminescence), electrophoresis, etc. In such display media, a technique using active driving elements (thin film transistor (TFT) elements) as image driving elements has been widely used in order to assure uniformity of screen brightness, a screen rewriting speed, and the like. For example, in a typical computer display, TFT elements are formed on a substrate, and liquid crystal, organic EL elements, or the like are sealed.
In this case, semiconductors such as amorphous silicon (a-Si) and polysilicon (p-Si) can be mainly used for the TFT elements. The TFT elements are fabricated by forming multiple layers of the Si semiconductors (and a metal film if necessary), and sequentially forming source, drain and gate electrodes on the substrate.
Moreover, formation of such TFT elements using a Si material involves a high-temperature process, adding a limitation on a substrate material that the substrate material needs to be resistant to the process temperature. Accordingly, a substrate made of a material having high heat-resistance, for example, a glass substrate needs to be used for practical applications. Note that a quartz substrate may be used, but the quartz substrate is expensive, which is economically problematic to increase the size of displays. Thus, as a substrate on which TFT elements are formed, a glass substrate is generally used.
However, a thin display formed by using the glass substrate is heavy, has poor flexibility, and may be broken if dropped. These characteristics are not desirable to satisfy the needs of portable thin displays which have risen with progress in information technology.
To satisfy the needs of light, thin displays, from the viewpoint of, for example, forming a flexible, light substrate, a semiconductor device (flexible semiconductor device) having TFT elements formed on a resin substrate (plastic substrate) has been developed.
For example, PATENT DOCUMENT 2 describes the technique of manufacturing a TFT element on a base material (e.g., a glass substrate) by substantially the same process as that conventionally used, and then peeling the TFT element from the glass substrate to transfer the TFT element on a resin substrate.
Moreover, PATENT DOCUMENT 3 describes the technique of directly forming a TFT element on a resin substrate.